This invention relates to metal casting apparatus, methods and molds and more particularly to the casting of metal in refractory, gas-permeable, shell-type molds which are lighter and have thinner wall thicknesses than the refractory, gas-permeable, shell-type molds commonly used in the ceramic shell casting process for lost wax casting of ferrous and nonferrous alloys such as steel, aluminum, and bronze.
Conventionally used refractory, gas-permeable, shell-type molds are made of multiple layers of ceramic slurry and sand. Thick-walled molds, consisting of upwards of 30 or more layers, are known in the industry. The ability to use lighter and thinner-walled molds (on the order of 5-8 layers or less) would be advantageous from both a cost and labor standpoint, especially when it is understood that these molds typically cannot be reused. Once a metal object has been cast in a mold, the mold is torn away from the cast metal object and discarded.
Traditionally used thick-walled refractory, gas-permeable, shell-type molds suffer functional disadvantage resulting from the fact that the molds have measurable but slight permeability. This characteristically low permeability prevents the mold cavity from filling out with molten metal in heavily detailed sections and in sections having large surface areas relative to volume because of entrapped air that cannot permeate out through the mold walls before the molten metal solidifies. Accordingly, it is desirable to use molds that yield more complete fill out of the mold cavities resulting in better capture of exact detail and close tolerances.
In the conventional ceramic shell casting process, it is possible to use one large mold cavity with a runner or runners that feed molten metal directly into the mold cavity. Alternatively, many smaller mold cavities can be connected to a main, central runner by feeder tubes commonly known as ingates. In instances where molten metal is fed in the vertical plane, the runner is commonly referred to as a down sprue. (The molten metal is poured through one or more pouring basins which in turn feed the molten metal into one or more down sprues.)
When large molds are being used, or when many molds are ganged along a runner or down sprue, molten metal may solidify before it completely fills the molds. This problem of non-fill can be solved by increasing the number of down sprues or ingates feeding each of the molds or by increasing the pouring temperature of the molten metal. Neither of these solutions is optimal. Using greater numbers of down sprues and ingates results in the need for additional retooling of the finished cast object in order to remove surface artifacts left by the down sprues or ingates with a commensurate increase in associated labor costs. An increased pouring temperature is undesirable because excess superheat can lead to very deleterious effects on the mechanical and structural properties of the finished cast objects including greater likelihood of void formation, microcracks, gross cracks and metal segregation.
Another problem associated with conventional ceramic shell casting processes resides in the fact that, when a mold cavity fills with molten metal, the pressure of the metallostatic head wants to burst the mold open. This is a common problem in the industry and there have been attempts to solve it by:
1. sinking the mold into a fluidized bed of sand (as molds get bigger and vertically higher, the more the sand covering the mold must weigh to offset the metallostatic head which wants to burst the mold open); PA1 2. making the walls of the molds very thick (one known method uses 30 or more coats of ceramic slurry and sand to make the shell of the mold); PA1 3. reinforcing the mold by building wire into the mold; or PA1 4. reinforcing the mold by adhering wire to the outside of the mold with refractory cement.
None of these methods is completely successful.
One method meeting with an additional degree of success for solving this mold rupture problem is a known vacuum casting method substantially described in U.S. Pat. No. 3,900,064 and using molds substantially as described therein and in U.S. Pat. No. 4,112,997. However, this known vacuum casting method is not completely successful either. According to that method, the mold is placed in a vacuum chamber and the entire vacuum chamber is suspended above a source of molten metal. A down sprue extends down into the molten metal. Vacuum is applied to the vacuum chamber, drawing metal into the mold through the down sprue. However, such a system which uses a mold in a vacuum chamber suspended over a molten metal source is constrained in the size castings that can be made due to a number of limitations.
The first limitation is the necessity to have a vacuum chamber large enough to enclose the mold. Very large vacuum chambers can be built to enclose large molds, but with difficulty.
The second limitation is even more difficult to overcome and is explained by comparison to conventional ceramic shell casting processes which do not employ vacuum. As explained above, in conventional ceramic shell casting processes, non-fill can be a problem when large molds are used or when many molds are ganged along a runner or down sprue. Attempts to remedy the non-fill problem, as explained above, include increasing the pouring temperature of the molten metal or using additional down sprues and ingates. In the known vacuum casting method, non-fill likewise can be a problem. Increasing the pouring temperature of the molten metal is an unacceptable solution because it can result in mechanical and structural defects as described previously. Furthermore, the use of multiple down sprues, as in the conventional ceramic shell casting processes, is difficult if not impossible to implement. This is because each down sprue is a ceramic tube which protrudes about 12-16 inches below the bottom of the vacuum chamber, and must pass through the wall of the vacuum chamber and connect to the mold in the vacuum chamber. This connection is complicated, requiring unerring precision, for molten metal will leak through the smallest hole. To locate multiple down sprues in a single chamber with the required level of precision would be difficult if not impossible. Furthermore, the labor costs associated with the achievement of such precision would be substantial.
There is no way to overcome the third problem associated with the known vacuum casting method. Using 14.7 psi as atmospheric pressure and taking 304 stainless steel as an example of the alloy being cast, one can quickly determine a maximum limit for how vertically high a casting can be made. The density of 304 stainless steel is about 0.28 lb/in.sup.3. At a pressure of zero in the chamber, the maximum column of metal that can be supported is 14.7.div.0.28 or about 52 inches. Of course, safety factors and practical considerations (e.g., one cannot realistically attain zero pressure) would result in a decrease of that number by at least about 20%.
The final limitation of the known vacuum casting method is that the entire vacuum chamber, with the down sprue protruding out of the bottom of the vacuum chamber, must be lifted and moved to the molten metal. And for large castings, the vacuum chamber must be held suspended over the molten metal, with the vacuum on, until the metal solidifies. (This solidification time is at least three to five minutes, depending on the alloy being cast.) Otherwise, as a result of gravity, the unsolidified metal will drain out of the mold cavity and back into the crucible which holds the molten metal. This can result in defects in the cast metal objects. The molds described in U.S. Pat. No. 4,112,997 represent an attempt to alleviate this problem.